October was the biotech sector's worst month in 2013. "Worst" is relative in an otherwise unbelievable year for biotech stocks, but as you can see from the chart above, our little corner of the market was beat by the S&P 500 in October. June was the only other month this year during which the S&P 500 outperformed the biotech sector, but the broader market just lost less. IBB data by YCharts There are many reasons for the recent weakness in biotech stocks. As the year ends, some investors have definitely decided to lighten up on their winners. Catalyst Pharmaceuticals ( CPRX) was the best-performing stock between January and the end of September. And here's what Catalyst looked like in October: ^NBI data by YCharts I probably had something to do with the Catalyst sell off. Acadia Pharmaceuticals ( ACAD) was the second-best performing stock January through September. And in October: ACAD data by YCharts Celldex Therapeutics ( CLDX): Third-best return January through September. Not in October: nd CLDX data by YCharts Picking up the pattern? Here's Biocryst Pharmaceutics ( BCRX), fourth best until Octoer: ^NBI data by YCharts -- Reported by Adam Feuerstein in Boston. Follow Adam Feuerstein on Twitter.